Permitted temperature for bending 304 Stainless Round Bar 1

[shsaeidian]

is there any standard, code, or reference for Permitted temperature for bending 304 Stainless Round Bar?
in the below thread has been mentioned that temp. should not go above 800F, is there a reference for this temp.?

thread367-249230

 

[EdStainless]

Above 800F you begin to run the risk of chrome carbide formation in grain boundaries (sensitization).
This is a common figure in virtually any SS reference.
While the graph infers that for low carbon material you can tolerate some time at higher temperatures, this assumes that the material was properly and fully solution annealed first (which usually isn't the case).


= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

Much depends upon the actual carbon content. 304 can be 0.04%C and it can be 0.08%C; there is a massive difference in sensitization risk between those. The TTT diagram shows that Carbon below 0.030% gives effective immunity during normal thermal operations like welding.

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

 

[EdStainless]

But, if there is any even a microscopic trace of Cr-C in the GB then you can cut the times in that graph by a factor of 100 or more. The reason is that that graph is based on the time to nucleation and growth to a certain extent. If there is even a trace of carbide still there from being under annealed at the mill (always) then you straight into the growth phase.
I have had 0.025% C material that would sensitize if you looked at it wrong because the mill anneal was crap. It would pass an A262 from the mill, but it wasn't truly solution annealed.
We used to tell people to limit it to 600F (assuming that they would push to 650F) and then you can be comfortable.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

Disclaimer: Graph is for comparison purposes only.

I think most of us down at the user end casually assume that solution anneals are equivalent and that they always do what they are supposed to.

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

 

[EdStainless]

I know IR, it is a situation that has gotten worse over the years.
In light gage material and tubing it isn't too bad. But in heavier material and pipe it is pronounced.
One issue is that the properties themself are not restrictive enough to encourage better annealing. Another factor is that A262 requires fairly extensive sensitization in order to fail the test.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[mfgenggear]

what is the stress relief temperature

 

[EdStainless]

There isn't really one. If you stay below risk of sensitization then you get almost not stress relief effect.
If material has been well annealed and is low C (<0.025%) I have resorted to using 1000F for 20min to get some strain relief, it is enough to reduce the magnetic signature and restore some ductility.
These are meta-stable alloys, though mostly austenitic but when strained they will have some transformation to martensite (no C but BCC) this greatly lowers ductility and makes them somewhat magnetic.
The only real fix for this is to go back to solution anneal (1950F) and rapidly cool (which causes stresses of its own).

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

EdS,

I have rejected more than one WPS claiming 'stress relief heat treatment' at 800°F for P-No. 8 material.
The common thread is that they were prepared by a CPWE - Certified Pretend Welding Engineer.

Also: a little knowledge is a dangerous thing. Especially when it comes to metallurgy.



"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

 

[EdStainless]

Yes, doing that to a weld will load the HAZ with chrome carbide, worst conditions.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

800°F is practically useless in effecting relief of stresses in 304. Besides that, as you say, only bad things can happen.

The PQR 'designer' chose the temperature because he recalled something about chromium carbides from his 2nd year Mat Sci course.

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."